1. Field of the Invention
The present invention relates to a backlight unit and a liquid crystal display using the backlight unit, and more particularly, to a backlight unit including a lamp guide and a liquid crystal display using the backlight unit.
2. Discussion of the Related Art
Liquid crystal displays have been used in notebook personal computers, office automatic equipments, audio/video equipments, etc. because of advantages such as miniaturization, thin profile, and low power consumption.
A liquid crystal display includes a liquid crystal display module and a backlight unit. The backlight unit may be classified into an edge type backlight unit and a direct-under type backlight unit depending on a location of a light source.
In the edge type backlight unit, a light source is positioned at an edge of a light guide plate. While light produced by the light source is repeatedly reflected inside the light guide plate, the light is emitted to the liquid crystal display module through a main surface of the light guide plate.
In the direct-under type backlight unit, a plurality of light sources is parallelly positioned under the liquid crystal display module, a reflective plate is positioned under the light sources, and a diffuser plate is positioned on the light sources. Light produced by the light sources is emitted to the liquid crystal display module through the diffuser plate.
Because the number of light sources in the direct-under type backlight unit increases as compared with the number of light sources in the edge type backlight unit, the direct-under type backlight unit can achieve a high luminance of an image.
Examples of lamps used in the direct-under type backlight unit include a cold cathode fluorescent lamp (CCFL), an external electrode fluorescent lamp (EEFL), and a light emitting diode (LED).
As the screen size of the liquid crystal display increases, lengths of the cold cathode fluorescent lamp and the external electrode fluorescent lamp become longer in proportion to the screen size of the liquid crystal display. However, because the cold cathode fluorescent lamp and the external electrode fluorescent lamp are manufactured in a cylinder shape having a very small diameter, the hang down phenomenon of the lamps occurs because of the weight of the lamps as the lengths of the lamps lengthen.
Further, as the screen size of the liquid crystal display increases, a size of the diffuser plate, that is positioned on the lamp to uniformly diffuse light coming from the lamp on the entire surface of the liquid crystal display panel, and sizes of optical sheets, that focuses light emitted from the diffuser plate on a liquid crystal display panel, increase. Therefore, a hang down phenomenon occurs in the diffuser plate and the optical sheets.
The backlight unit emits light having the non-uniform luminance distribution and non-uniform optical characteristics to the liquid crystal display panel because of the lamps, the diffuser plate, and the optical sheets in which the hang down phenomenon partially occurs. Hence, the display quality of the liquid crystal display is reduced.
A related art backlight unit, as shown in FIGS. 7 and 8, includes a lamp guide positioned on a hang down portion so as to prevent the hang down phenomenon.
FIG. 7 is a perspective view of a related art lamp guide, and FIG. 8 is a cross-sectional view schematically showing a liquid crystal display including a backlight unit including the related art lamp guide shown in FIG. 7.
As shown in FIGS. 7 and 8, the related art backlight unit includes a lower cover 10, a reflective plate 20, a lamp guide 30, a plurality of lamps 40, and an optical member 50 including a diffuser plate 51 and a plurality of optical sheets 53.
The lower cover 10 has a receptacle structure capable of receiving the plurality of lamps 40 in an inside space of the lower cover 10, and the reflective plate 20 is positioned on a surface of the inside space.
As described above, the lamps 40 are formed in a cylinder shape of a cold cathode fluorescent lamp or an external electrode fluorescent lamp to produce light. The light produced by the lamps 40 is incident on the diffuser plate 51 by the reflective plate 20.
The diffuser plate 51 includes a plurality of beads and scatters the light using the beads.
The optical sheets 53 include-one or more diffuser sheets and one or more prism sheets. The optical sheets 53 uniformly emit the light coming from the diffuser plate 51 on the entire display surface of a liquid crystal display panel 60, and focus the light on the entire display surface by changing a traveling path of the light in a direction perpendicular to the display surface of the liquid crystal display panel 60.
The lamp guide 30 is used to hold the two or more lamps and includes holder units 3 1, a first support unit 35, a second support unit 33, and a fastening unit 37.
The holder unit 31 surrounds a circular portion of the lamp 40 and has an opening into which the lamp 40 is inserted.
The lamp guide 30 includes at least one second support unit 33. The second support unit 33 has a cone shape so that an area of the second support unit 33 contacting the diffuser plate 51 is minimized.
The first support unit 35 is formed in the form of a pedestal so that the lamp guide 30 is supported in the inside space of the lower cover 10 in which the reflective plate 20 is formed. The holder units 31 and the second support unit 33 are formed on the first support unit 35. Because the two or more holder units 31 are formed on the first support unit 35 so as to hold the different lamps 40, the first support unit 35 extends in a direction perpendicular to a longitudinal direction of the lamps 40.
The fastening unit 37 is inserted into a hole (not shown) passing through the lower cover 10 and the reflective plate 20 to fasten the lamp guide 30 to the lower cover 10 and the reflective plate 20.
The related art backlight unit can solve the hang-down phenomenon of the lamp 40 and the hang-down phenomenon of the optical member 50 including the diffuser plate 51 and the optical sheets 53 using the lamp guide 30.
The lamp guide 30 is formed of a reflective material capable of reflecting the light produced by the lamp 40. In case the lamp guide 30 does not reflect the light produced by the lamp 40, Mura occurs at a formation position of the lamp guide 30.
However, the lamp guide 30 is generally formed of a material whose a reflectance is smaller than a reflectance of the reflective plate 20. Therefore, in an initial drive stage of the lamp 40, the amount of light reflected by the lamp guide 30 is relatively less than the amount of light reflected by the reflective plate 20 and the amount of direct light generated in the lamp 40. Hence, Mura occurs at the formation position of the lamp guide 30.
As above, although the lamp guide is formed of the reflective material, Mura is not prevented from occurring in the initial drive stage of the lamp.